The present invention is related to a coating formulation, for application to mechanical devices or components having surfaces associated with or exposed to combustion and surfaces subjected to elevated temperatures by a chemical grafting technique, to prevent the deposition, at high temperatures, of carbon or coke, which can impact the performance of such devices.
In recent years, carbon deposition has become a serious problem for surfaces associated with combustion, such as fuel injectors, as well as surfaces subjected to elevated temperatures, such as turbine engines and other fuel supply components. This problem can be caused by increased fuel inlet temperatures and a higher compressor air temperatures in, e.g., gas turbines. In high-performance aircraft engines, for instance, the fuel inlet temperature might exceed 350xc2x0 F. and the compressor discharge temperature could be as high as 1,600xc2x0 F. Under certain operating conditions, the wetted-wall temperature quickly reaches between 400xc2x0 F. and 800xc2x0 F. Combined with high temperatures and slow velocities, the fuel may undergo chemical degradation, which could lead to the formation of carbon deposits within a short time. Similar deposition problems can occur in automotive applications, power generation, industrial or residential furnaces, and other similar applications.
Once carbon deposits begin to form, they contribute to a number of problems such as reduced fuel flow rate, excessive fuel pressure drop, sticking valves, blocked strainers or filters, and poor spray quality. Any one of these problems could easily affect the overall combustor performance and result in high maintenance costs.
Today""s fuel injectors must often operate in a high-temperature environment without carbon formation for long hours. This requirement would be very difficult to meet without significant design improvements. Based on field experience and research studies, certain design guidelines are followed to avoid the flow or surface conditions that facilitate carbon formation. For example, the wetted-wall temperature is usually kept below 400xc2x0 F., and the fuel inlet temperature must be lower than 225xc2x0 F. Because fuel velocity also plays an important role in determining carbon formation, the flow rate range must be carefully examined. It has been found that carbon formation is most severe for combustors operating under soak back and steady state conditions with flow velocities ranging between 2 and 4 m/s. As fuel velocity exceeds 6 m/s, however, carbon deposition becomes less likely due to effective heat transfer and short resident times.
Despite these guidelines, engineers still have to rely on other design considerations to meet stringent requirements on injector durability and service life. These considerations include pre-stressing the fuel, using fuel additives, applying carbon-resistant coatings, providing better surface finish, adding more effective insulators, using ceramic materials, and implementing passive or active cooling. Based on a literature search and an in-depth study, the use of carbon-resistant coatings on the metal surface appears to be one of the most effective and economical means of reducing carbon formation inside fuel injector passages under adverse temperature environments.
A number of patents describe techniques for preventing the deposition of carbon onto nozzle surfaces. For example, U.S. Pat. No. 6,123,273, to Loprenzo, et al., discloses a dual fuel nozzle for inhibiting carbon deposition onto combustible surfaces in a gas turbine, in which the dual fuel nozzles produce an accelerated swirling air flow to preclude impingement of oil spray droplets onto the metal surfaces of the nozzle, and hence prevent carbon deposition thereon. U.S. Pat. No. 5,315,822, to Edwards, teaches coating fuel-wetted elements for gas turbines, where the high temperature alloys have a layer of titanium carbide, titanium nitrite, titanium boride, or mixtures thereof on them to inhibit a formation of carbon or coke. U.S. Pat. No. 5,336,560, to Spence, et al., teaches gas turbine elements which are protected from carbon deposition by the application of a coating of alumina and silica from a sol gel formulated for the purposes of creating an acceptable coating composition. U.S. Pat. No. 6,145,763 to Fleming et al. teaches coating automotive fuel injectors with a fluorine-containing amorphous hydrogenated carbon film coating to resist the formation of carbonaceous deposits. This reduction in coking and deposits improves the fuel economy and engine performance.
There are various coating techniques for improving the physical properties of stainless steel and alloy material for carbon resistance. These techniques include thermal spraying, detonation-applied refractory, chemical vapor deposition, and ion implantation. These techniques generally are complicated, expensive, and limited to specific applications.
The present invention is the discovery of a coating formulation for application to surfaces associated with combustion, such as fuel injectors, and surfaces subjected to elevated temperatures, such as fuel supply components, to prevent the deposition at high temperatures of carbon or graphite on such surfaces, such as stainless steel and alloyed steel compositions. The coating formulation forms a thin layer of polymer coating on the surface by chemical grafting involving the use of a graft initiator to create active bonding sites, on the metal surface, for the silicone-based prepolymer to undergo polymerization on the metal surface. The thin layer of polymer coating prevents the deposition at high temperatures, i.e., temperatures of from 300xc2x0 F. and up to 700xc2x0 F., of carbon or graphite on surfaces such as a stainless steel or stainless steel alloys. The bonding and polymerization are completed in a single application process without using complex equipment. The coating and process helps surfaces associated with combustion and surfaces subjected to elevated temperature reduce carbon or coke formation and thereby improves the durability and performance of those surfaces.
The coating is easy to apply, does not add any significant dimensions to the surface, since it is in the range of about 0.0001 to about 0.010 inch thick, but could also be about 0.0005 to about 0.003 inch thick, and cannot be easily removed. The coating process does not require expensive equipment or cause any environmental or safety hazards. The coating process can be used for parts or components with complicated configurations. Further, the coating can be used for any surfaces where the surfaces will encounter coking or carbon deposition problems, such as aircraft fuel injectors, automotive fuel injectors, fuel injectors for gas turbine power generators, and the like.